1. Field of the Invention
The present invention relates to novel powder coating binders with a glass temperature of ≧35° C. containing at least one organic polymer and one or more radiation-curing reactive thinners, a process for the production of these powder coating binders and their use as a component of powder coatings.
2. Description of the Prior Art
Powder coating binders are applied on to a substrate, melted with heat and cured by radiation-curing, electromagnetic radiation. Curing by means of electromagnetic radiation has gained increasing importance in recent years. The curing process is very rapid, which reduces the process speed of the coating operation and thus represents a cost saving. Radiation-curing powder coatings have already been known for a very long time (U.S. Pat. No. 3,974,303) and are usually based on polyesters (e.g. DE-A 42 28 514), polyurethanes (EP-A 410 242) and polyacrylates (DE-A 44 13 436). Mixtures are also known.
The use of oligo- and polyurethanes as powdered binders makes it possible to obtain particularly high-quality coatings. While EP-A 410 242 describes the production of these binders in organic solvents, DE-A 19 939 843 teaches the solvent-free production using a high proportion by weight of 10 to 95% of a crystalline component made up of a mono- or diisocyanate and a monohydroxyalkyl (meth)acrylate. This synthesis without solvents is only made possible by the viscosity-lowering effect of the molten, crystalline component.
A disadvantage of the oligo- and polyurethanes described in EP-A 410 242 and DE-A 19 939 843 is their poor grindability. The products can only be ground without forming lumps at temperatures below 0° C. and all the other powder handling must take place at temperatures below room temperature.
In the case of amorphous materials, their properties are determined by whether the material is investigated above or below its glass transition temperature. Below its glass transition temperature, grindable products are generally obtained. If the glass transition temperature is too high, the products can be stirred only at very high temperatures. The properties of a powder coating are determined to a decisive extent by the ambient temperatures occurring during powder coating. In summer, and particularly in the vicinity of the ovens needed to melt the powder coatings, higher temperatures than conventional room temperatures of 20 to 25° C. can be expected. In practice, therefore, a sufficiently high glass transition temperature of ≧35° C. is necessary for powder coating.
In the PCT application WO 03/044111, UV powder coating binders are described which are amorphous and of particularly low viscosity in the molten state. For some applications, the high molecular weight of the urethane acrylate is disadvantageous here. Thus, hardly any degrees of freedom remain to adjust the double bond density.
In UV liquid paint chemistry it is conventional to use so-called reactive thinners, on the one hand to lower the viscosity of the liquid paint and at the same time to improve the UV reactivity by increasing the double bond density, which is generally also associated with improved paint resistance values.
The object on which the invention is based therefore consisted in providing radiation-curing powders that can be ground at room temperature, have a long shelf life and have no tendency towards caking under production conditions and practice-related processing conditions, which can be produced without solvents and, in contrast to the prior art, contain reactive thinners conventionally used to adjust the double bond density in liquid paint chemistry.